1. Field of the Invention
This invention relates to static electrical power converters utilizing naturally commutated power circuits having positive and negative thyristor banks, and more particularly to a method and apparatus for bank selection in such converters.
2. Prior Art
A broad class of electrical apparatus known as static power frequency changers synthesize an output waveform of a desired stable or variable frequency from a multiphase AC voltage source of a different or varying frequency. A control circuit sequentially turns on a plurality of power switches connected to the individual phases of the source voltage to generate a number of component waveforms which are summed to produce the desired output waveform. One type of power switch commonly used in these frequency changers is the thyristor (also known as the silicon controlled rectifier or SCR) which, when properly biased, will conduct current in a forward direction in response to a firing pulse applied to a gate electrode and will continue to conduct current in the forward direction despite termination of the firing pulse until the flow of current is interrupted for a preset interval. The thyristors are commonly arranged in these frequency changers for natural commutation; that is, the firing pulses are generated at instants such that the phase voltage applied to the oncoming thyristor is sufficient to commutate off the previously fired thyristor. Thus only properly phased firing pulses need be generated by the control circuit since turn off of the thyristors is inherent in the arrangement.
Examples of frequency changers using naturally commutated thyristor power circuits are dual converters and cycloconverters. The dual converter produces a controlled DC output signal which may be positive or negative while the cycloconverter generates a desired AC output waveform. Since both of these types of converters must generate output currents of both polarities and since the thyristor can only conduct current in one direction, the thyristors are arranged in positive and negative banks which carry the positive and negative load currents respectively.
Firing of the thyristors in a bank may, or may not, be inhibited when the load current is supplied by the other bank. If the thyristors in both banks are fired continuously, without any change to the basic firing control which is normally optimized to produce the best possible output waveform, a large magnitude interbank circulating current will develop which imposes an increased load on the converter and the voltage source. In the past, techniques employed to reduce the circulating current have shifted the optimized firing instants, leading to increased waveform distortion. Commonly owned, copending patent application of Stacey, et al., Ser. No. 095,820 filed concurrently herewith suggests that only the firing instants of the nonload carrying bank be shifted, thereby reducing the circulating current while producing an inherently better output waveform.
If firing of the thyristors in each bank is alternately enabled and inhibited appropriately in phase and synchronism with the load current, no continuous circulating current will be developed. This method of controlling the firing of the thyristor banks is commonly known as bank selection. In the basic form of bank selection, firing of the thyristors in the outgoing bank is inhibited and firing of those in the incoming bank is enabled as the output current of the converter passes through zero. Various techniques which have been developed for determining when to transfer the load from one bank to the other are discussed in Thyristor Phase-Controlled Converters & Cycloconverters, B. R. Pelly, Wiley-Interscience 1971, pages 114 to 126 and 198 to 203. U.S. Pat. No. 3,568,033 discloses apparatus for transferring the load from one bank to the other upon the occurrence of zero crossing points of the fundamental component of the AC output current waveform of a cycloconverter. U.S. Pat. No. 3,852,654 discloses an improvement in the apparatus of U.S. Pat. 3,568,033 which takes into account distortion due to load transients when determining the zero crossing points.
The prior art approaches to bank selection work well in converters operating from a source having more than adequate voltage, and the current is transferred smoothly from one bank to the other. Unfortunately, this smooth transition of current will not always take place if the source voltage is reduced to the minimum level required to fabricate the fundamental output waveform. Under these conditions, the current in the last conducting thyristors of the outgoing bank will sometimes not be driven to zero after their firing is inhibited. This has been determined to especially be a problem in converters in which the thyristors in each bank are divided into groups coupled to each other by interphase transformers. In this arrangement large fault currents can be developed with conventional bank selection in a manner to be discussed below.
It is common in converters utilizing bank selection to employ the controlled firing pulse overlap technique discussed at pages 190-198 in the Pelly book mentioned above. Under this technique, firing pulses are applied to both the positive and negative banks, thus permitting circulating current to flow, so long as the load current is instantaneously less than a prescribed level. Whenever the load current exceeds this "threshold" level, the firing pulses are automatically removed from the nonload carrying bank and the circulating current is inhibited. Under this technique, however, inhibiting of the firing pulses in the nonload carrying bank is not directly related to the actual condition of the bank and all of the thyristors in the bank are inhibited at the same time even where they are divided into groups. It is still possible under this technique for large fault currents to develop.
It is a primary object of the present invention to provide static converters having naturally commutated thyristor power circuits which operate without large interbank circulating currents and without the possibility of large fault currents.
It is another object of the invention to achieve these results using improved bank selection techniques.
It is a more particular object of the invention to provide bank selection wherein firing pulses to the thyristors in the nonload carrying bank are inhibited only after the current through these thyristors has actually gone to zero.
It is still another object of the invention to inhibit firing pulses independently to each thyristor group in the nonload carrying bank.